Section 287.020 RSMo.(2000), defines accident as an unexpected or unforeseen event or series of events that occur suddenly, without fault, and produce objective symptoms of an injury. The injury must be "clearly work related", and that term is defined as work being a substantial factor in the resulting medical condition. Further, an injury is not compensable merely because work was a triggering or precipitating factor. To be medically causally related the work must be a substantial factor in the cause of the resulting medical condition or disability. §287.020.2 RSMo A causative factor may be substantial even if it is not the primary or most significant factor. Cahall v.Cahall, 963 S.W.2d 368, 372 (Mo.App. 1998) (overruled on other grounds). Further, there is no minimum percentage set out in the Workers' Compensation Law defining "substantial factor." Id. Whether employment is a substantial factor in causing the injury is a question of fact. Sanderson v. Porta-Fab Corp., 989 S.W.2d 599, 603 (Mo.App. 1999) (overruled on other grounds). Determinations of this kind require the assistance of expert medical testimony. Medical causation not within lay understanding or experience requires expert medical evidence. Wright v. Sports Associated, Inc., 887 S.W.2d 596 (Mo.banc 1994) (overruled on other grounds).
Claimant bears the burden of proof in establishing the essential elements of her case. Establishing accident and medical causation are essential elements. For Employer to be held liable, Claimant must produce evidence which reasonably links her injury to the alleged work accident. Griggs v. A.B. Chance Co., 503 S.W.2d 697, 704 (Mo.App. 1973). Claimant has failed to meet her burden. According to the medical records, Claimant believes a hepatitis A vaccine administered by Employer in 2003-2004 caused her subsequent knee problems. This belief is clearly refuted by her personal physician, Dr. Uchiyama, and Claimant offers no contrary expert medical opinion. As accident and medical causation are not established, all other issues presented are moot.